It will NEVER happen. Why would Marvel tie their cinematic universe into that of a completely different studio's? A studio they have no power over? A studio that can do whatever they want with the character(s)?

What happens if/when Fox decide to reboot instead of a sequel? Whilst Marvel keep their current universe (with recasts but essentially the same continuity)? There's a big discrepancy there.

Or Fox could turn around and kill off their Quicksilver if they like! It would be stupid on Marvel's part.